Bikini perhaps left line after caution regularly ignored (5)
I believe the answer is:
atoll
'bikini perhaps' is the definition.
(Bikini Atoll)
'left line after caution regularly ignored' is the wordplay.
'left' becomes 'l' (common abbreviation**).
'line' becomes 'l'.
'after' says to put letters next to each other.
'regularly ignored' means one should take alternating letters.
The alternating letters of 'caution' are 'ato'.
'l'+'l'='ll'
'll' after 'ato' is 'ATOLL'.
(Other definitions for atoll that I've seen before include "Pacific coral island" , "Coral island(s) surrounding a lagoon" , "String of islands" , "Circular coral reef" , "Coral circle" .)